Study On Material Erosion Mechanism Of Micro EDM Based On Marangoni Effect

As a branch of micro machining technology,micro electrical discharge machining(micro EDM)technology can process any conductive material without being affected by the mechanical properties such as material strength and hardness.Therefore,it exhibits unique advantages in the precision machining of metal materials.Micro electrical discharge machining has been widely used in the fields of precision molds,aerospace,biotechnology,medicine,sensors,and microelectronic device manufacturing.However,due to the randomness and complexity of the process of micro electrical discharge and material removal,the machining mechanism has not yet been fully clarified.Micro EDM is the result of multiple single pulse discharge processes,so studying the single pulse discharge characteristics in micro EDM is a simple and effective method to explore the material erosion mechanism of micro EDM.This paper proposes the Marangoni effect in micro EDM and systematically studies the mechanism of micro EDM based on the Marangoni effect.The main contents are as follows:The discharge process and material erosion mechanism of micro EDM are theoretically analyzed,including dielectric breakdown,characteristics of discharge channels,deionization and other machining processes during micro EDM,as well as the explosive force erosion theory,thermogenic force erosion theory,and hydrodynamic erosion theory in micro EDM.A single pulse discharge experiment of micro electric spark was conducted,and the variation rules of discharge crater radius,depth,and volume were analyzed based on the discharge duration.The results showed that with the increase of discharge duration,the radius of the discharge craters first increased rapidly and then slowly,and the volume of the discharge craters continued to increase with the increase of discharge duration.However,the depth of the discharge craters did not significantly increase with the increase of discharge duration,but fluctuated in a small range.The distribution of discharge energy in micro EDM was studied.Based on the experimental results of single pulse micro electrical discharge,the energy fraction introduced into the workpiece during micro EDM was calculated by comparing the measured and theoretical values of the crater size,taking into account the effects of melting latent heat,vaporization latent heat,and recast layer.The results show that the energy fraction introduced into the workpiece increases with the increase of the discharge duration,ranging from 0.63～20.5 μs,the energy distribution range transmitted to the workpiece is 7.1 ～16.3%;As the discharge duration increases,the radius of the discharge crater is always smaller than the radius of the discharge plasma and the radius of the heat affected zone.The radius of the discharge channel increases rapidly,then slowly,and finally reaches a relatively dynamic equilibrium state.Finite element simulation of micro EDM was carried out.Firstly,a comprehensive thermal model of single pulse discharge for micro EDM is established,and a Gaussian distributed heat source,plasma channel expansion empirical formula and thermophysical properties of materials with temperature change are introduced into the model.The simulation results of the temperature field and stress field at the discharge point are analyzed and discussed,and it is found that the temperature at the discharge point of the workpiece is very high,and the high temperature is mainly concentrated in a very small area,and the outward temperature gradually decreases along the direction of the radius of the discharge crater,and the distribution law of thermal stress and the distribution law of temperature are basically the same,the thermal stress is the largest at the center of the discharge channel,but the thermal stress decreases rapidly along the radius and depth of the discharge crater.Then,based on the comprehensive thermal model,the Marangoni thermal convection effect is introduced into the modeling of micro EDM,and the multiphysics coupling simulation is carried out by using heat transfer,laminar flow and dynamic mesh modules in COMSOL Multiphysics simulation software.The result analysis shows that the temperature gradient on the surface of the discharge crater melt pool causes the surface tension of the metal material to change,and this surface tension gradient would cause the metal liquid in the molten state in the discharge crater to flow,and after the discharge ends,the metal liquid in the molten state would cool and solidify,and finally form a half-bowl-shaped discharge crater.The size of the discharge crater obtained by the multiphysics coupling simulation was compared with the crater size measured in the single-pulse discharge experiment,and it was found that the formation process and morphology of the discharge crater could be more vividly described by introducing the Marangoni effect,and the Marangoni effect played an important role in the formation of the discharge crater.Therefore,by introducing the Marangoni effect,the simulation accuracy of micro EDM can be further improved,which is conducive to accurate prediction of processing results.